1. Field of the Invention
The invention relates to a motor control apparatus, and an electric power steering apparatus including the motor control apparatus.
2. Description of the Related Art
An electric power steering apparatus that provides a steering assist force to a steering mechanism of a vehicle by driving an electric motor in accordance with a steering torque applied to a steering wheel by a driver has been used. A brush motor has been widely used as the electric motor of the electric power steering apparatus. Recently, a brushless motor has also been used as the electric motor of the electric power steering apparatus, from the viewpoints of improvement of reliability and durability, a decrease in inertia, and the like.
To control a torque generated by the motor, a motor control apparatus detects a current flowing into the motor, and executes a feedback control based on the difference between a current to be supplied to the motor and the detected current. Generally, the control is executed by appropriately setting voltages applied to phases of the motor by turning on/off a plurality of switching elements included in a switching circuit that drives the motor (typically by using the duty ratios of PWM signals). In the motor control apparatus, a voltage applied to the switching circuit that drives the motor, typically a power supply voltage is measured, and the above-described control is executed based on the measured voltage.
Japanese Patent Application Publication No. 2001-278086 (JP-A-2001-278086) describes the configuration in which a power supply voltage and voltages at both terminals of a motor are measured, and the terminal voltages of the motor are corrected based on the measured values when steering is assisted. Also, Japanese Patent Application Publication No. 2005-170225 (JP-A-2005-170225) describes the configuration in which the average voltage of a battery and a peak current flowing into the motor are measured, and a corrected current value is calculated based on the measured values.
In each of the technologies described in Japanese Patent Application Publication No. 2001-278086 and Japanese Patent Application Publication No. 2005-170225, correction for each phase of the motor is not performed. Therefore, control accuracy may not be sufficiently high. For example, in the switching circuit that drives the motor, the voltage may drop due to a shunt resistance and a wiring. Therefore, the voltage applied to the motor may be different from the power supply voltage. Accordingly, particularly when a large current flows into the motor, the difference between the current to be supplied to each phase of the motor and the current that actually flows into the phase of the motor increases due to the resistance. Therefore, the control accuracy decreases.